1. Field of the Invention
The present invention concerns an x-ray anode with a highly thermally stressable surface, as well as a method to produce such an x-ray anode.
2. Description of the Prior Art
X-ray anodes in conventional x-ray tubes have a surface that, due to its intended purpose, is exposed to a significant thermal stress that varies over a large range. To generate the x-ray radiation, the surface of such an x-ray anode is struck by a beam of high-energy electrons. Upon deceleration of the electrons in the surface of the x-ray anode, the desired x-ray radiation results. At the point on the surface of the x-ray anode on which the electron beam strikes, known as the focal spot, temperatures of up to 2500° C. occur. In order to increase the lifespan of the x-ray tubes, in many cases rotating anode tubes are used in which a plate-like x-ray anode rotates around its axis of symmetry. The electron beam strikes this rotating anode in the radial outer region, meaning close to the circumference of the anode plate. Due to the rotation of the anode plate, the surface continuously moves away from underneath the focal spot that is fixed within the x-ray tube, such a focal path on the anode plate moves under the focal spot and the electrons do not always strike at the same location on the surface of the anode plate.
FIG. 2 shows, in an enlarged section, the more precise assembly of a conventional rotating anode, as well as the temperature ratios along the focal path surface. The rotating anode has a plate 4, for example made of Mo or TZM, on which a focal path layer 3, made of tungsten with an additive of rhenium (WRe) is located at the outer circumference. In larger rotating anodes, a layer 5 of graphite is often bonded with the plate 4 in order to increase the heat storage capacity. The focal spot B moves on the surface 2 of this focal path layer 3. The rotation direction of the anode plate is designated by the arrow direction R. During operation, an average temperature of approximately 1000° C. exists in the focal path layer 3. On the focal path surface 2, meaning in the first μm of the focal path layer 3, the temperature is approximately 1,500° C. Given one rotation under the focal spot B, the temperature curve at a specific surface point is indicated. Immediately upon being swept over by the electron focal spot B, the temperature at this location rises to approximately 2500° C. Thereafter, the temperature cools relatively quickly back to 2000° C., and then falls gradually to 1500° C., until finally the focal spot B sweeps over the appertaining point of the focal path surface 2 again.
The relatively powerful thermal shook when the electron beam, with its high energy density, sweeps over the focal path surface 2 leads to a thermal fatigue that results in a severe roughening of the focal path surface 2. FIG. 4 shows, in schematic view, a strongly magnified section through such a surface. Molar-like accumulations form between individual tears 11, such that a wavy roughening of the surface results. FIG. 3 shows a microscopic exposure of a part of the surface of a focal path of a conventional rotating anode plate at the end of its “lifetime” meaning at the time it must be taken out of service. The image area corresponds to approximately 2.64 mm2. This microscopic image shows very clearly that melted droplets have formed on the surface, which project from the focal path like stalagmites, as well as thermal shock tears existing between the melted droplets. Individual cracks are designated by white arrows. In a more precise evaluation of the microscopic exposure shown in FIG. 3, in total of 194 crack formations were counted in 2.64 mm2. Such cracks can grow to approximately 0.7 mm deep down into the surface. Due to the tear growth, the possibility is increased that particles will be released into the high-voltage space, the probability of high-voltage disruptions increasing as a result.
Furthermore, the increasing surface roughness of the focal path surface 2 leads to a reduction of the radiation yield. For explanation, reference is made to FIGS. 2 and 5. Typically, the focal path surface 2 is inclined outwardly at a small angle α of approximately 7° relative to the surface of the anode plate 1. Upon the electron beam striking the focal path surface 2, x-ray radiation is simultaneously emitted in all directions. An x-ray beam hemisphere 8 shown in FIG. 1 results over the focal path surface 2. Only a small part of the total emitted x-ray radiation is actually used as wanted x-ray radiation 10 directed to the examination subject; the largest part is radiated in other directions and is gated by a housing or by means of a diaphragm. As FIG. 1 shows, only the portion of the x-ray radiation 14 radiated very shallowly over the focal path surface 2 outwardly is used. This geometric arrangement has the advantage that the focal spot can be kept relatively large (normally approximately 10 mm×1 mm), in order to keep the load for the focal path surface as small as possible. Due to the use of only the portion of the x-ray radiation radiated shallowly outwardly over the plate surface, the effective size of the x-ray source point (which is a projection of the actual focal spot B located on the focal path surface 2 on the plane of the diaphragm 9) is reduced to smaller dimensions of, for example, 1 mm×1 mm. Due to the shrinking of the “effective x-ray source point”, a better resolution is achieved in the x-ray exposure. FIG. 5 shows how the surface roughening leads to a radiation reduction in the direction of the used portion 10 of the x-ray radiation 14. While some rays 14 just manage to reach, via the diaphragm, over the roughened surface 2 to the examination subject, in contrast to this closely surface-proximal x-ray radiation 13 is screened by the roughness on the surface 2. By means of long-term tests, it has been shown that, given a typical load of a standard anode plate with 60 kW electron shots, at the end of its lifetime the surface roughness values can be 45 μm (RZ), and this can cause a weakening of the wanted x-ray radiation by 14% and more. Since the portion of the wanted x-ray radiation is relatively small in relation to the total generated x-ray radiation anyway, it would be of great advantage. Particularly for use in a computed tomography apparatus, to prevent an additional dosage loss of over 10% and more due to a focal path deterioration.
The deterioration of the focal path is directly dependent on the extent of the power acting on the surface. Therefore, previous methods have aimed at extending the lifespan of x-ray anodes predominantly by reducing this power. A possibility theoretically exists to increase the rotation speed of the anode plate so that the focal path surface moves more quickly under the focal spot. Since the load of the surface can be reduced only with the square root of the rotation speed, however, the limit of the effective improvement methods is reached relatively quickly. A necessary quadrupling of the rotation speed in order to halve the load of the anode surface is, due to the carrier load, unrealistic given normal carrier design. This is true both with the use of ball bearings and with the use of floating bearings to journal the anode plate. A further attempt to reduce the focal path load is a dosage modulation in CT systems, in which the radiation dosage is always lowered by 20% when a patient is irradiated from the front or back. Although the deterioration is thereby temporally delayed, it is not prevented. Furthermore, for specific applications, a desired further increase of the pulse power (a short-duration increase of the x-ray output power) inevitably leads to an increased deterioration of the focal path.